Nickel-base alloys both cast and forged are extensively used in the design of turbine components requiring weldability and high temperature capabilities, particularly those alloys providing a good combination of strength and ductility.
High-strength nickel-base superalloys, which usually contain aluminum and titanium as the major hardening elements are strengthened by the precipitation of gamma prime (.gamma.') phase with ordered fcc structure. When aluminum and titanium are partially or completely replaced by niobium or tantalum, a different precipitation phase can be produced having the ordered bct structure designated as gamma double prime (.gamma."). These .gamma."-strengthened alloy systems provide remarkably good tensile properties to intermediate temperatures.
Inconel 718 (IN 718), also referred to herein as the "base alloy", contains 25% by volume, more or less, of the .gamma." phase as well as a small amount of ordered fcc .gamma.' precipitates. Investigations utilizing transmission electron microscopy have established that coherent .gamma." precipitates are in disc-shape morphology with a {100} habit plane and have a cubic-cubic orientation relationship with the fcc matrix. More detailed characteristics of the phase chemistries of .gamma.' and .gamma." are given in "Phase Chemistries in Precipitation-Strengthening Superalloy" by E.L. Hall, Y.M. Kouh, and K.M. Chang [to appear in Proc. Electron Microscopy Society of America, Aug. 1983]. The chemical combination of IN 718 alloy is set forth in TABLE I.
TABLE I ______________________________________ Element wt % at % ______________________________________ Ni bal. bal. Cr 18.6 20.7 Fe 18.5 19.2 Mo 3.1 1.9 Nb 5.0 3.1 Ti 0.9 1.1 Al 0.4 0.9 C 0.04 0.19 ______________________________________
Despite the relatively low volume fraction of strengthening phase (.about.25%) therein, IN 718 alloy, when forged and heat treated, has a room temperature yield strength of 165 ksi, which is higher than that of Udimet 700 (.about.140 ksi), which contains 45 volume % .gamma.' precipitate. This unique strength characteristic is responsible for the extensive use of IN 718 alloy in many turbine engine applications.
In addition to its strength and ductility capabilities, another notable property of IN 718 alloy is its excellent weldability, a characteristic which is apparently related to the sluggish precipitation kinetics of the coherent .gamma." strengthening phase. This characteristic is of particular importance, because some welding processes are mandatory in the manufacture and repair of certain turbine engine components. Most precipitation-hardening superalloys, when welded, develop cracks in the heat affected zone and in the weld metal during welding or during post-weld heat treatment. Cracking accompanying the welding operation or subsequent heat treatment causes excessive and costly reworking of welded components and prevents optimum design latitude for components requiring joining during fabrication. IN 718 alloy is known to be the only non-susceptible alloy that also provides adequate strength. It is for that reason that IN 718 has been selected as the base alloy against which improvement is to be measured herein.
Unfortunately, the tensile strength of IN 718 alloy is relatively sensitive to temperature compared to conventional .gamma.' strengthened alloys. Further, the stress rupture life of IN 718 deteriorates rapidly at temperatures in excess of 1200.degree. F. There is a continuing demand for new high-strength weldable, castable, forgeable superalloys having improved temperature capability for operation above 1200.degree. F., because of the continuing increase in the turbine engine operating temperature.
The problem of providing weldability in a nickel-base cast alloy is addressed in U.S. Pat. No. 4,336,312 - Clark et al. In accordance With the Clark et al. invention, conventional nickel-base castable superalloys are modified by reducing the aluminum content and increasing the carbon content thereof. In addition, as-cast modified nickel-base alloy components are subjected to a pre-weld thermal conditioning cycle, which is believed by the patentees to result in a precipitate that retains adequate ductility within the grains.
U.S. Pat. No. 3,046,108 - Eiselstein is directed to a malleable, age-hardenable, nickel-chromium base alloy in which the emphasis is on the presence of "controlled and coordinated amounts of alloying elements" (column 1, lines 45 and 46). The composition of IN 718 lies within the teachings of this patent. The exclusion of iron, the inclusion of tantalum and the inclusion of cobalt are merely options.
Certain terminology and relationships will be utilized herein to describe this invention, particularly with respect to the precipitation hardening elements such as aluminum, titanium, tantalum and niobium. The approximate conversions of weight percent to atomic percent for nickel-base superalloys are set forth as follows:
Aluminum (wt %).times.2.1=Aluminum (at %) PA1 Titanium (wt %).times.1.2=Titanium (at %) PA1 Niobium (wt %).times.0.66=Niobium (at %) PA1 Tantalum (wt %).times.0.33=Tantalum (at %) PA1 "at % TOTAL" is the term representing the total content of aluminum, titanium, niobium and tantalum expressed in atomic percent. PA1 "R.sub.gdp " is the value of the sum of the niobium and tantalum contents (in at %) divided by at % TOTAL. When this value is 0.62 or greater .gamma." is the only precipitation strengthening phase present.
The following are definitions useful in understanding this invention:
The following U.S. patents disclose various nickel-base alloy compositions: U.S. Pat. No. 2,570,193; U.S. Pat. No. 2,621,122U.S. Pat. No. 3,061,426; U.S. Pat. No. 3,151,981; U.S. Pat. No. 3,166,412; U.S. Pat. No. 3,322,534; U.S. Pat. No. 3,343,950; U.S. Pat. No. 3,575,734; U.S. Pat. No. 4,207,098 and U.S. Pat. No. 4,336,312. The aforementioned U.S. patents are representative of the many alloying situations reported to date in which many of the same elements are combined to achieve distinctly different functional relationships between the elements such that phases providing the alloy system with different physical and mechanical characteristics are formed. Nevertheless, despite the large amount of data available concerning the nickel-base alloys, it is still not possible for the metallurgist to predict accurately the physical and mechanical properties of a new combination of known elements even though such combination may fall within broad, generalized teachings in the art.